Method of making field effect layers

ABSTRACT

A charge controlled viewing storage tube uses an insulating film to store charge and a field-effect layer adjacent a phosphor layer for its switching function. Such field-effect layer is now made easier to fabricate in large areas by making it in the form of tiny flakes that are sprinkled onto a phosphor during the latter&#39;&#39;s curing stage.

nited States Patent [191 Cowher et al.

[11 3,862,858 1 1 Jan. 28, 1975 METHOD OF MAKING FIELD EFFECT LAYERS [75] Inventors: Melvyn E. Cowher, Katonah;

Thomas O. Sedgwick, Crompond, both of NY.

[73] Assignee: International Business Machines Corporation, Armonk, NY.

[22] Filed: Mar. 16, 1973 [21] Appl. No.: 341,821

[52] U.S. Cl. ..117/215,1l7/33.5 E [51] Int. Cl. B44c 1/00, B44d 1/02 [58] Field of Search 117/215, 333.5 E, 33.5 C,

[ 56] References Cited UNITED STATES PATENTS 3,650,824 3/1972 Szepesi ct al 117/335 E Primary Examiner-John D. Welsh Armrney, Agent, or FirmGeorge Baron 1571 ABSTRACT ing stage.

5 Claims, 7 Drawing Figures PATENTED JAN28 I975 8.882.85 SHEET 10F 2 FIG. 1

PRIOR ART PATENTED 3,862,858

sum 2 BF 2 FIG.4B

FIG.5

METHOD OF MAKING FIELD EFFECT LAYERS BACKGROUND OF THE INVENTION In the Proceedings of the IEEE of Oct. 1968, pp. 1716-7, an article by B. Kazan and .l. S. Winslow ap peared, entitled Viewing Storage Tubes Based Upon Field-Effect Conductivity Control. The primary elements of such field-effect storage tube comprise, on its viewing face, a glass faceplate provided at its inner surface with a set of fine transparent conducting strips that are interdigitally connected to an a.c. voltage source. This inner surface is coated in sequence with an electroluminescent phosphor layer, a semiconductor, and an insulating film. In writing, a charge pattern is established on the surface of the insulator by the electron beam in the tube, producing a corresponding conduction pattern in the adjacent semiconductor film by field-effect action. The phosphor glows and produces an image wherever the conductivity of the field-effect layer is high. Unlike image storage panels which retain surface charges in the form of negative oxygen ions on a ZnO semiconductor field-effect layer, the presently described storage-tube screen employs an insulating film on the semiconductor to retain charges.

It has been found that a storage tube with certain advantages in stability can be made using SiO /poly Si/- SiO- as the field-effect-insulator layer combination. However, it is not possible to deposit these layers sequentially directly on the phosphor because the temperature of the process will degrade either the SiO poly Si/SiO layers or the phosphor. Consequently, the SiO /poly Si/SiO must be fabricatedseparately from the phosphor and then bonded to the phosphor.

It has been found that the above noted storage tube can be fabricated when the display area is relatively small. However, if large screens are desired, the fieldeffect layer that is composed of a triple layer of SiO;/- poly Si/SiO and which is only two microns thick, tends to break away from the phosphor layer. However, even in the small area device subsequent required etching steps induce a high stress between the field-effect layer and the phosphor layer, causing the two layers to separate.

It has been found, however, that if the triple layer of SiO /poly Si/SiO is made in the form of many tiny flakes rather than as a single continuous layer, this separation is avoided. Such flakes would be only 2p. thick and be no larger than 20 mils and would be sprinkled onto the phosphor layer when the latter is wet, or in its uncured state. Manners of making such flakes will be described hereinafter in detail, but the major thrust of the invention is to enable the formation ofa large area device consisting of the very thin field-effect layer needed in the making of the storage tube of the type shown and described in the above noted Kazan et al article, by using a large number of overlapping flakes which can be fabricated without stress problems and without the inherent fragility of a very thin continuous large area film.

The foregoing and other objects, features and advantages of the invention will be apparent from the following more particular description of the preferred embodiments of the invention as illustrated in the accompanying drawings.

DESCRIPTION OF THE DRAWINGS FIG. 1 is a cross-section of the prior art field-effect storage tube with a portion of the tube blown-up, as shown in FIG. 1A.

FIG. 2 is a showing of one method for making the flakes of SiO /poly Si/SiO that will be attached to a phosphor screen.

FIG. 3 is a showing of yet another method for making such flakes.

FIG. 4A is an enlarged edge view and FIG. 4B is an enlarged top view of the flakes used in forming the field-effect layer.

FIG. 5 is a view of the method of attaching the flakes to the phosphor screen.

As seen in FIG. I, the prior art device whose operation is sought to be improved is a storage tube 2, having the conventional electron beam deflectng apparatus 4 for causing an electron beam E to selectively dwell on the face 6 of such tube 2. The face 6 consists of an outer glass plate 8 which supports, on its inner face, electrodes 10 that are transparent and are deposited with a density of 50 lines per inch. A 2 mil thick layer 12 of electroluminescent phosphor, such as ZnS in a glass binder, is deposited over the: electrodes 10 onto glass face 8. Phosphor layer 12 is coated with a semiconductor film 14 of the order of l-2p. thick, and polysilicon is an example of a suitable semiconductor. The final film is a 0.5;]. thick insulating film 16, such as SiO Collector grid 18 and bias supply 20 for electrodes 10 are standard equipment for the storage tube 2.

The description of the manner in which information is stored, observed and erased is discussed in the above noted Kazan et al., article and, as such, does not form part of this invention. It has been found, however, that difficulty arises in the making of a large area, but very fragile l-2u thick field-effect layer 14, and in the bonding of said layer 14 to the phosphor layer 12.

This invention avoids such difficulties in the formation of the prior art storage tube by making very small flakes of triple-layered material, namely, SiO /poly Si/- SiO and then broadcasting them over a large area of tacky, uncured phosphor.

To prepare the flakes that will be attached to the ZnS phosphor layer 12, one starts (See FIG. 2) with a silicon wafer 19. The top surface of such wafer is oxidized to a thickness of 0.5 1. to attain the SiO layer 21. Over this layer 21, polycrystalline silicon 23 is chemically vapor deposited by pyrolysis of silane, at a temperature of 650C, to a thickness of l-Su, with the lower values, i.e., I or 2a, of the range being preferred. The top surface of the polycrystalline silicon is oxidized to a thickness of 0.5,u, to attain SiO layer 25. Upon completion of the multilayerunit of SiO /poly Si/SiO the Si wafer 19 is preferentially etched away from its superimposed layers 21, 23 and 25, using a solution of ethylene diamine and pyrocatechol and water. The multilayers deposited over the Si wafer 19 are highly stressed and thin so when such multilayers 21, 23 and 25 are separated from the Si wafer 19 by the etching step, the multilayers break up into very minute particles 26 (See FIG. 4). Such particles 26 are about 2,u thick and have a representative, but not a limiting length, ranging from 20 mils down to 2 mils.

To apply these flakes of SiO poly Si/SiO to a ZnS phosphor, the latter is deposited (See FIG. 5) onto its glass face 8 as a tacky layer 12 of ZnS or equivalent electroluminescent material. The flakes 26 are sprayed. broadcast, or sent through a sieve: (not shown), onto the ZnS layer 12 and the latter will cure at about 90C after a few hours. The flakes that are deposited will overlap two adjacent drive lines such that electrical continuity between them exists for a.c. current. lt is to be understood that different electroluminescents will cure at different times and at different temperatures, and the person practicing this invention will modify the curing step accordingly.

In a second method (See FIG. 3) of making the particles or flakes, a l2p. thick polycrystalline silicon layer 22 is grown on a quartz or glass substrate 28 by conventional and well-known techniques. After growth, the thin layer of polysilicon, because of stresses within it will, upon etching in a hydrofluoric acid bath 30, break up into particles of polysilicon 32. The acid 30 carrying particles 32 is sent through filter 34, whereby the acid is captured in container 36 and the particles 32 are retained in filter 34. These fine particles 32 are passed through an oxidizing furnace (not shown) and collected after oxidation in a suitable receptacle. Such oxidized particles consist of the layers SiO /poly Si/SiO Such fine particles can then be applied to an electroluminescent in its uncured state, to be fused to the latter electroluminescent when it is cured.

It has been found that the flakes 26, being so thin, have negligible capacitance so there is no interference with their field-effect function in the operation of the storage tube 2 providing the electrical continuity to a.c.

' current noted hereinabove is maintained.

The use of flakes of SiOz/POIy Si/SiO need not be limited to electroluminescent display tubes. Such particles may be used with displays where the light-emitting layer or surface is a liquid crystal. In general, the invention is easy to implement, and is particularly applicable to the manufacture of electroluminescent screens that require large, but very thin, layers of a field-effect material. The above described procedure removes much of the critical handling of materials needed heretofore to make storage tubes requiring large area field-effect layers.

What is claimed is: 1. In a method for adhering large area field-effect layers onto an electroluminescent material that is cured to a final hardness but is wet or tacky in its uncured state comprising the steps of: providing a substrate, depositing an electroluminescent layer on said substrate while said layer is .in its tacky uncured state,

broadcasting flakes of field-effect material, of the order of two microns thick, onto said electroluminescent material, and

curing said electroluminescent material so as to permanently affix said flakes to said electroluminescent material.

2. In the method of claim 1 wherein said field-effect flakes consist of oxidized polycrystalline silicon.

3. In the method of claim 1 wherein said electroluminescent layer is ZnS.

4. ln a method for improving the operation of a display-storage tube including a transparent member having spaced apart transparent electrodes thereon and comprising the face of said tube including the steps of:

depositing an electroluminescent layer that is cured to a final hardness but is wet and tacky in its uncured state on said transparent member, while said electroluminescent layer is in its tacky uncured state,

broadcasting flakes of field-effect material onto said uncured electroluminescent layer, and

curing said electroluminescent layer so as to permanently affix said flakes to said electroluminescent layer.

5. In the method of claim 4 wherein said field-effect flakes are composed of a three-layered material consisting of a first layer of silicon dioxide, a second layer of polycrystalline silicon and a third layer of silicon di- 

2. In the method of claim 1 wherein said field-effect flakes consist of oxidized polycrystalline silicon.
 3. In the method of claim 1 wherein said electroluminescent layer is ZnS.
 4. In a method for improving the operation of a display-storage tube including a transparent member having spaced apart transparent electrodes thereon and comprising the face of said tube including the steps of: depositing an electroluminescent layer that is cured to a final hardness but is wet and tacky in its uncured state on said transparent member, while said electroluminescent layer is in its tacky uncured state, broadcasting flakes of field-effect material onto said uncured electroluminescent layer, and curing said electroluminescent layer so as to permanently affix said flakes to said electroluminescent layer.
 5. In the method of claim 4 wherein said field-effect flakes are composed of a three-layered material consisting of a first layer of silicon dioxide, a second layer of polycrystalline silicon and a third layer of silicon dioxide. 